Additives to negative photoresists which increase the sensitivity thereof

ABSTRACT

THE USE OF A SCANNING ELECTRON BEAM TO GENERATE A PATTERN IN A NEGATIVE PHOTORESIST IS KNOWN. ELECTRON BEAM EQIPMENT CAN BE MADE WHICH IS CAPABLE OF SCANNING VERY QUICKLY, BUT STANDARD NEGATIVE PHOTORESISTS REQUIRE SUCH A LARGE FLUX OF ELECTRONS FOR PROPER EXPOSURE THAT THE SCANNING EQUIPMENT MUST BE OPERATED AT SPEEDS SUBSTANTIALLY SLOWER THAN THE CAPABILITY OF THE EQUIPMENT. BY ADDING CERTAIN COMPOUNDS WHICH DISSOCIATE READILY INTO FREE RADICALS TO THE PHOTORESIST, THE SENSITIVITY OR SPEED OF THE PHOTORESIST IS EFFECTIVELY INCREASED. AS A RESULT, THE ELECTRON BEAM CAN SCAN AT A HIGHER RATE. COMPOUNDS WHICH ARE MOST EFFECTIVE ARE BENZOPHENONE, BENZIL AND 1,4-DIPHENYL-1,3-BUTADIENE.

April 30, 1974 a. BROYDE 3,808,155

ADDITIVIES TO NEGATIVE PHOTORESISTS WHICH INCREASE THE RESIST THICKNESSSENSITIVITY THEREOF I Filed March 26, 1973 PARTIALLY CYCLIZED CISPOLYISOPRENE a I.O% BENZOPHENONE PARTIALLY CYCLIZED CIS POLYISOPRENERESIST THICKNESS (M) Tlg: Q.

RESIST THICKNESS (U) 05 PARTIALLY CYCLIZED c|s POLYISOPRENEY a I.0BENZIL 0.4

PARTIALLY CYCLIZED 'CIS POLYISOPRENE I I I I l I I l l di 4 s 8 I0l 2l4l 6 DOSE DENSITY (,U COULOMBS/ cm PARTIALLY CYCLIZED CIS POLYISOPRENE am I,4DlPHENYL-I,3BUTADIENE I 0.2 PARTIALLY CYCLIZED CIS POLYISOPRENE-O.| I I' I I I I I o 2 4 6 8 IO I2 I4 l6 DOSE DENSITY (,U C0ULOMBS/CM2)United States Patent 3,808,155 ADDITIVES T0 NEGATIVE PHOTORESISTS WHICHINCREASE THE SENSITIVITY THEREOF Barret Broyde, Lawrence Township,Mercer County, N.J., assignor to Western Electric Company, Incorporated,New York, N.Y. Continuation-impart of application Ser. No. 137,032, Apr.23, 1971, which is a continuation of application Ser. No. 764,866, Oct.3, 1968, both now abandoned. This application Mar. 26, 1973, Ser. No.344,790

Int. Cl. G03c 1/68, N70

US. Cl. 252-500 6 Claims ABSTRACT OF THE DISCLOSURE The use of ascanning electron beam to generate a pattern in a negative photoresistis known. Electron beam equipment can be made which is capable ofscanning very quickly, but standard negative photoresists require such alarge flux of electrons for proper exposure that the scanning equipmentmust be operated at speeds substantially slower than the capability ofthe equipment. By adding certain compounds which dissociate readily intofree radicals to the photoresist, the sensitivity or speed of thephotoresist is effectively increased. As a result, the electron beam canscan at a higher rate. Compounds which are most efiective arebenzophenone, benzil and 1,4-diphenyl-1,3-butadiene.

CROSS-REFERENCE TO RELATED APPLICATION This is a continuation-in-part ofmy copending application, Ser. No. 137,032, filed Apr. 23, 1971, nowabandoned, said application being a continuation of my application Ser.No. 764,866, filed Oct. 3, 1968, now abandoned. Said copendingapplication is assigned to the same assigned as the instant application.

BACKGROUND OF THE INVENTION (1) Field of the invention This inventionrelates generally to additives to negative photoresists which increasethe sensitivity thereof and, more particularly, the invention relates toadditives to standard negative photoresists which result in increasedreactivity of the photoresist. The invention has particular applicationin, but is not limited to, the generation of microminiature circuitpatterns by electron beam exposure of negative photoresists.

A negative photoresist is an organic material which, when exposed toradiation, undergoes chemical reactions of the type referred to ascrosslinking, which reactions result in insolubilizing the exposedphotoresist. The crosslinking reactions are of the type that can beinitiated either by light or by electrons. Because it is possible togenerate electron beams of substantial energy but only 1.0; or smallerdiameter, their use in the generation of extremely small circuitpatterns is preferred to the use of light. Electron beams also have amuch better resolution capability than is possible when using an opticalmask and light exposure, and they have a much greater depth of focus.The exposure of a conventional positive photoresist involvessolubilization of the exposed areas, and the chemical reactions involvedarea of the scission or degradation type, which also requireabsorption'of light or electrons. Because this type of photoresistrequires higher flux densities for proper exposure than negativephotoresists require, electron beams are not widely employed in thisservice. Materials that have been successfully used aselectron-sensitive positive photoresists are discussed by Haller et al.,IBM Journal, May 1968, pp. 251-256.

3,808,155 Patented Apr. 30, 1974 The most common negative photoresist incurrent use are Kodak Photoresist (KPR, KPRZ, KPR3, trademarked productsof Eastman Kodak Company) and Kodak Thin Film Resist (KTFR, trademarkproduct of Eastman Kodak Company). The KPR composition is based on thedimerization of polyvinyl cinnamate, while KTFR is based on thecrosslinking of a polymerized isoprene dimer, i.e., partially cyclizedcis-polyisoprene, averaging one double bond per 10 carbon atoms. Anothermember of the KPR group besides KPR 2 and .3, is KOR (trademark forKodak Ortho Resist). Another product, KMER (trademark for Kodak MetalEtch Resist) belongs to the KTFR group. The invention will be describedwith primary reference to use of polyvinyl cinnamate and partiallycyclized cis-polyisoprene, but it will be appreciated that it is not solimited.

The crosslinking and insolubilization of resists is a complexphenomenon, but is believed to be describable, broadly, as follows. Apolyvinyl cinnamate or KPR-type resist has the following generalformula:

TCI-Irr l I The number average molecular weight N.A.M.W.) is180,000-230,000, and the weight average molecular weight (W.A.M.W.) is315,000-350,000. Upon exposure to light or electron energy, a diradicalis formed:

(urem a-IA).

where A is the vinyl cinnamate monomer (structure 1 where n=1). Thediradical then reacts with diradical to form a 4- member ring:

t nant-G m) wreak-Loin I another,

Further excitation and dimerization leads to an insoluble product; nofree radicals participate in these reactions.

The partially cyclized cis-polyisoprene or KTFR-type resists can becharacterized as follows:

CH1 CH8 oom-o-o 1 11 H --OCHz \l J CH: :1 (4) These materials (averagingone double bond per 10 carbon atoms) have a N.A.M.W. of 65,000i5,000 anda W.-A.M.W. of about 120,000, and are insolubilized by free radicalreactions. Thus, radiation produces a di radical:

CH: CH; ).-CH:-'-

II --CCH1 where B is the monomer of (4). The diradical reacts with othermolecules until the free radical terminates. For good resolution,additives may be incorporate to keep the chain short. In all of theabove structural formulae, the subscripts (n, m, p, s, I) refer tointegers which are determinative of molecular weight. While polyvinylcinnamate and partially cyclized cis-polyisoprene are insolubilized bydifferent mechanisms, both result in crosslinked systems.

The procedures for generating a microminiature pattern circuit byelectron bombardment of a photoresist are well established, and aresummarized briefly below. The substrate is typically an oxidized siliconwafer or a chromium-coated glass plate. The photoresist is dissolved ina suitable solvent and applied to the substrate, which may then be spunat a high speed to leave an even film of the photoresist, having acontrolled thickness, on the substrate surface. Alternatively, thephotoresistsolvent solution may be sprayed on. In either case, most ofthe solvent evaporates immediately. The photoresistcoated substrate isthen dried or baked briefly to drive 05 any remaining solvent and toimprove adhesion. The coated substrate is then placed in a vacuumchamber and, when the vacuum has been established, it is radiated in thedesired pattern and with an appropriate dosage. The coated and radiatedsubstrate is then placed in a developer, which is a solvent for thesoluble portion of the resist, to dissolve and remove the unexposedportions. It is again dried or baked. The desired pattern area on thesubstrate is now free of any covering film, and etching, plating oroxidizing follows. After this step, the remaining resist is strippedoff.

There are a variety of limitations imposed upon the radiation step, butthese are fully convered in the prior art (listed below) andneed only besummarized here.

Briefly, the amount of radiation must fully expose the photoresist allthe way down to the substrate, or else the developed photoresist willfloat off when the underlying, undeveloped photoresist is dissolved inthe developer. n the other hand, too much radiation will cause strippingproblems and even polymer degradation. The amount of radiation necessaryto form an insoluble photoresist is a function of the molecular weightof the material, and the gross amount of radiation. The efficiency ofthe crosslinking reactions is related to the accelerating potential ofthe electrons, penetration range (also a function of potential) andother factors. For instance, it has been determined that the maximumfilm thickness that can be developed by kv. electrons is about 6,500 A.,and by kv. electrons is about 2 On the other hand, photoresists shouldinitially be at least 6,000 A. thick to avoid pinhole problens (a 6,990A- filnt wi l to about 4,000 A. when developed). Other limitations whichmust be considered (2) Discussion of the prior art Prior workers havecarried out extensive studies on the foregoing limitations, particularlywith respect to the sensitivity and resolution capability of standardresists. This work need not be described herein, but is referenced belowfor background information:

Thornley et al., Electron Beam Exposure of Photoresists, Journal of theElectrochemical Society, vol. 112, No. 11, November 1965, pp. 1151-1153;

Broers, Combined Electron and Ion Beam Process for Microelectronics,Microelectronics and Reliability, vol. 4, 1965, pp. 103-104;

Kanaya et al., Measurement of Spot Size and Current Density Distributionof Electron Probes by Using Electron Beam Exposure of Kodak PhotoresistFilms, Zeit. f. Lichtund Elektroninoptik, vol. 25, No. 5, 1967, p. 31;and

Matta, High Resolution Electron Beam Exposure of Photoresists,Electrochemical Technology, vol. 5, No. 7-8, July-August 1967, pp.382-385.

None of these prior workers have made any effort to alter conventionalphotoresist compositions, although it is significant to note thatThornley et al. appreciated the problems which they pose: For serialexposures, such as may be required in printed circuit generators, themaximum exposure rates are limited by the sensitivities of presentlyavailable resists. (Thornley et al., op cit, p. 1151).

While prior workers who have studied electron beam development ofresists to generate small patterns have worked only with the availableresists, workers in the field of photolithography, where photoresistswere first employed, have proposed literally thousands of compounds asphotopolymerization initiators, catalyzers and sensitizers. The end inview was generally to increase the sensitivity or resolution of thephotoresist to light of a particular wavelength. This work is notreadily summarized, but the following US. patents are consideredrepresentative: 2,816,091; 2,831,768; 2,861,057; 3,168,404; 3,178,283;3,257,664; and 3,331,761.

OBJECTS OF THE INVENTION A general object of the present invention is toprovide new and improved additives to negative photoresists whichincrease the sensitivity thereof to electrons.

A further object of the present invention is to provide additives tostandard negative photoresists which result in increased reactivity ofthe photoresist itself.

Another object of the present invention is to improve the sensitivity ofa standard negative photoresist by including novel additives therein.

A further object of the present invention is to reduce the flux densityand, hence, the exposure time required to fully expose a standardphotoresist, by incorporating novel additives therein.

Various other objects and advantages of the invention will become clearfrom the following detailed description of several embodiments thereof,and the novel features of the invention will be particularly pointed outin connection with the appended claims.

THE DRAWINGS FIGS. l-3 are plots of resist thickness vs. flux densityfor exposure of 6,000 A. films of partially cyclized cispolyisoprene andpartially cyclized cis-polyisoprene plus the preferred additives of theinvention.

SUMMARY AND DESCRIPTION OF EMBODIMENTS In essence, the present inventioncomprises the addition, to a resist-solvent solution (polyvinylcinnamate photoresist-solvent solution or p rti y y ized si -p vis)prene photoresist-solvent solution), in small amounts, of compoundswhich readily dissociate into free radicals. These enhance thecrosslinking of the polyvinyl cinnamate and the partially cyclizedcis-polyisoprene, thus insolubilizing them. There are many compoundswhich will do this, but most have undesirable side effects, such ascausing crosslinking in the dark, without any exposure. Many peroxidesand hydroperoxides fall into this category. Three compounds have proveneffective; they are:

benzophenone OBH- -CBH The amount of the additive used is important. Iftoo little additive is present, sufficient free radicals will not begenerated to cause a maximum effect. On the other hand, if too much ofthe additive is present, the free radicals will react with each otherrather than with the resist, and crosslinking will not be aided. It hasbeen determined that no more than about 5% (all percentages are weightpercent) of the additive should be added to either the polyvinylcinnamate resist-solvent mixture, or the partially cyclizedcis-polyisoprene resist-solvent mixture. It should be understood,however, that this may amount to 20% or even 50% of the respectiveresist after the solvent is removed. Generally, a 1% solution of theadditive is preferred.

If one knows the average molecular weight of the photoresist film andthe electron accelerating potential, and makes certain assumptionsregarding electron penetration, scatter and energy transfer, the geldose of energy can be calculated from theory (the gel dose is theelectron fiux necessary to record an image in the film surface, i.e.,the minimum dose to cause insolubility). Experimental results are infair agreement with such calculations. When an additive causes a largenumber of free radicals to be formed at each collision of an electronwith a molecule, then it is not unreasonable to expect that the numberof molecules crosslinked at each such energy transfer site will behigher. The problem, as noted above, is to add compounds that will notreact spontaneously. While increased crosslinking could be expected withproper additive selection, the magnitude of improvement achieved withthe above-noted compounds is quite remarkable. In particular, afive-fold reduction in the dose density required to fully expose a 4,000A. film is achieved. The improvement is not linear; the gel dose isreduced little if at all by using the additives. These facts are allclear in the following specific examples.

A further requirement of the additive is that it be soluble in thesolvent system employed with the particular resist. The three notedcompounds satisfy this requirement.

Both polyvinyl cinnamate and partially cyclized cispolyisoprene aredissolved in a solvent thereof. With the latter, a thinner may also beemployed; this acts merely to reduce viscosity and produce a thinnerfilm. The solvent system used for polyvinyl cinnamate is 86-87%chlorobenzene and 13-14% cyclohexanone. The partially cyclizedcis-polyisoprene solvent system is 12% ethylbenzene, 82% mixed xylenesand 6% methylcellosolve. Both systems also contain a sensitizer; inpartially cyclized cispolyisoprene (commercially available as KTFR thisis believed to be 2,6-bis(p-azidobenzilidene)4-methylcyclohexanone. Thepartially cyclized cispolyisoprene thinner is primarily mixed xylenes.

EXAMPLE I To establish a basis for comparison, tests were first madewith polyvinyl cinnamate photoresists without any additives. A polyvinylcinnamate resist-solvent solution was applied to a chromium-coated glassplate. The resistsolvent solution was commercially obtained andcomprised polyvinyl cinnamate (N.A.M.W. of 180,000 to 230,000; W.A.M.W.of 315,000 to 350,000) dissolved in 8687% chlorobenzene, and 13-14%cyclohexanone. The coated glass plate was then spun so that theresulting coating, after baking at C. for 10 minutes, was 6,000 A.thick. The coated plate was then placed in a vacuum chamber and radiatedwith electrons accelerated at 15 kv. The plate was developed with apolyvinyl cinnamate developer, commercially obtained, and baked at 150C. for 10 minutes. The following results were obtained:

(a) Flux needed to record an image (gel dose) =1.1 10- coul./cm.

(b) Flux needed to form 3,000 A. thick resist layer -6 10- coul./cm.

(c) Flux needed to form maximum thickness (after development, 4,000 A.)resist=10 10* coul./cm.

EXAMPLE II To establish a basis for comparison, tests were first madewith partially cyclized cis-polyisoprene photoresist without anyadditives. A partially cyclized cis-polyisoprene photoresist-solventsolution was mixed with a thinner (mixed xylenes) in a 1 to 3 ratio. Theresistsolvent solution was commercially obtained and comprised partiallycyclized cis-polyisoprene (averaging one double bond per 10 carbonatoms; N.A.M.W. of 65,000 $5,000; W.A.M.W. of about 120,000) dissolvedin 12% ethylbenzene, 82% mixed xylenes and 6% methylcellosolve. Themixture was applied to a chromium-coated glass plate (or, alternatively,to a silicon slice onto which a 18,000 A. SiO layer had been grown), andthen spun to a thickness of 8,000 A. After baking at 150 C. for 10minutes, the film was 6,000 A. thick. The coated plates were then putinto a vacuum chamber and radiated with 15 kv. electrons. The plate wasdeveloped with a partially cyclized cis-polyisoprene developer,commercially obtained, and a partially cyclized sis-polyisoprene rinse,commercially obtained, and baked at 150 C. for 10 minutes. The followingresults were found:

(a) Flux needed to record image (gel dose)'==0.9 10- coul./cm.

(b) Flux needed to form 3,000 A. fil1n=4 10- coul./cm.

(c) Flux needed to form maximum (4,000 A.) thickness=7.5 X 10- cou1./cm.

Under identical conditions, but with 5 kv. electrons, the dose densitiesrequired to expose partially cyclized cispolyisoprene films were:

(a) 0.5 X 10'" coul./cm. (b) 0.75 X10" couL/cmf (c) 2x10- coul./cm.

EXAMPLE III The procedure of Example I was repeated except that a 5weight percent solution of benzophenone in the polyvinyl cinnamateresist-solvent solution was prepared and employed. The three tests notedin Example I were carried out (with 15 kv. electrons). The results wereas follows:

(a) 1.0 10 coul./cm. (b) 1.5 10- cou1./cm. (c) 2.0 10- couL/cmf".

EXAMPLE IV The procedure of Example II was repeated except that a 1weight percent solution of benzophenone in the thinned (1 to 3)partially cyclized sis-polyisoprene photoresist-solvent solution wasprepared and employed. Dose densities for the three tests with 15 kv.electrons were as follows:

(a) 0.5 10- couL/cm. (b) 1.0 10 coul./cm. (c) 1.75 coul./crn.

The improvement achieved by this additive is graphically illustrated inFIG. 1.

If 5 kv. electrons are used instead of kv. electrons, results for thethree tests are as follows:

(a) 0.45 X 10-- coul./cm. (b) 0.5 10 coul./cm. (c) 0.75 X 10 coul./cm.

Reasons for the higher efliciency of lower-energy electrons, and reasonsfor preferring 15 kv. beams, are discussed below.

EXAMPLE V The procedure of Example II was repeated except that a oneweight percent solution of benzil in the thinned partially cyclizedcis-polyisoprene resist-solvent solution was prepared and employed.Results of the three tests are as follows:

(a) 0.5 X 10- coul./cm. (b) 1.0 10 coul./cm. (c) 1.5 10- coul./cm.

The improvement achieved with this additive is graphically illustratedin FIG. 2.

EXAMPLE VI The procedure of Example II was repeated except that a 0.1weight percent solution of 1,4-diphenyl-1,3 butadiene in the thinnedpartially cyclized cis-polyisoprene photoresist-solvent solution wasprepared and employed. Dose densities for the three tests were asfollows:

(a) 0.5 10- couL/cm. (b) 1.5 10- coul./cm. (c) 1.5 10- coul./cm.2.

The improvement achieved with this additive is illustrated in FIG. 3.

The magnitude of improvement brought about by each of the additives isreadily seen in Table I, where the percent reduction in dose for each ofthe three levels, as compared to the photoresist without any additives,is set forth.

TABLE 1.

Reduction in dose, percent Example (2.) Gel dose (0) 3,000 A. (0) 4,000A.

It will be noted that one of the effects of the additives of the presentinvention is to increase the slope of the plot of resist thickness vs.dose density to near infinity near the gel point (see FIGS. 1-3). Byusing the minimum dose density needed to achieve the desired thickness,backscattered electrons or scattered primary electrons are minimized ifnot eliminated, and resolution capability of the resist iscorrespondingly increased. Underthese conditions, an edge definition ofabout 300 A. can be expected as an upper limit. This is significantlybetter than previously reported definition.

It will be further noted by comparing the partially cyclizedcis-polyisoprene radiated with 5 and 15 kv. electrons, that the 5 kv.samples required less energy at all three stages. It is quite true, infact, that lower energy electrons act much more efficiently than higherenergy electrons; on the average, about 2.5 times the number ofmolecules at each energy transfer point will react at 5 kv. than will at15 kv. It would seem appropriate, then, to utilize lower energyelectrons, but control of the size of the beam is more difficult at lowenergies. If very high potentials are used (+20 kv.) the Cfi'ICiIICY ofcrosslinking drops too low and back-scatter can become a significantproblem. For these reasons, a 15 kv. accelerating potential ispreferred.

It is to be understood that various changes in the details, steps,materials and arrangements of parts, which have been herein describedand illustrated in order to explain the nature of the invention, may bemade by those skilled in the art within the principle and scope of theinvention as defined in the appended claims and their equivalents.

What is claimed is:

1. An electron sensitive photoresist composition comprising a partiallycyclized cis-polyisoprene resist and a sensitizing compound comprising1,4-diphenyl-1,3-butadiene, said resist and said compound beingdissolved in a solvent system employed with said resist, theconcentration of said compound in the photoresist-solvent system beingan amount ranging from 0.1 to 5%.

2. The composition as claimed in claim 1 wherein said resist and saidcompound are dissolved in a solvent system comprising 12% ethylbenzene,82% mixed xylenes and 6% methylcellosolve.

3. A method of increasing the electron sensitivity of a photoresistcomprising a partially cyclized cis-polyisoprene resist which comprisescombining the photoresist with a sensitizing compound comprising1,4-diphenyl-1,3- butadiene.

4. The method as defined in claim 3 wherein said resist and saidcompound are dissolved in a solvent system employed with said resist.

5. The method as defined in claim 4 wherein said compound is present insaid photoresist-solvent system in an amount ranging from 0.1 to 5%.

6. The method as defined in claim 4 wherein:

said solvent system comprises 12% ethyl benzene, 82%

mixed xylenes and 6% methylcellosolve.

References Cited UNITED STATES PATENTS 3,529,960 9/1970 Sloan 96-115 R2,670,286 2/1954 Minsk et a1. 96-1 15 R 2,670,2875 2/1954 Minsk et a1.961l5 R 3,594,243 7/1971 Deutsch et a1. 96--35.1

OTHER REFERENCES Robertson, E. M., et al., Journal of Applied PolymerScience, vol. II, issue No. 6, pp. 308-311 (1959).

Kosar, J.,Light-Sensitive Systems, 1967, pp. -147 and -167.

Thornley, R. F. M., et al., I. of the Electrochemical Soc., vol. 112,No. 11, November 1965, pp. 1151-1153.

RONALD H. SMITH, Primary Examiner US. Cl. X.R.

